This invention relates to an automatic exposure control device of the stop-predetermined type for a single lens reflex camera wherein information concerning the scene brightness is memorized and the stop is controlled automatically by an electromagnetic mechanism.
Heretofore, it has been known in the art to automatically control the stop size of a camera, in accordance with the scene brightness, by a meter clamping method. However, the use of a meter is disadvantageous because of its lack of strength and reliability. Further, since the meter is also used as an exposure meter, it is inconvenient to use the meter in combination with other elements. Also the exposure meter occupies a large space. On the other hand, a balancing method which employs a servo-motor is disadvantageous in that the motor requires excessive power and is too large to be conveniently built into a camera body without enlarging the camera body size.
Where the shutter-preferential system is employed to control the iris opening, time is the output. In APEX operation, if the brightness value (Bv) varies in three steps, the time (Tv) also varies in three steps but must be a value proportional to 2.sup.3.
More specifically, if time is exponentially related to the brightness value and, for example, in lenses having f stops from 1.4 to 22 in eight steps, it would take 1 ms to stop down to f2 and 1 .times. 2.sup.8 = 256 ms to stop down to f22. As is known, in single lens reflex cameras the amount of light must be adjusted before the shutter is actuated. Accordingly, if it takes an excessively long period of time for such automatic adjustment, one would not only miss an opportunity for correctly timed shutter operation but the stability of photography operation itself would be impaired. For these reasons, it has not been suitable to use aperture time control with the shutter-preferential system.